"It was about Mach 9.6," said engineer Randy Vorland after Tuesday's flight. "We performed pretty much like we expected.

He added: "I think it's easier than people think it is. We can really do this stuff. I don't mean to make it sound too easy, but it's definitely doable."

Wing trip

Scramjets are being developed in a number of countries as an alternative propulsion system to rockets.

Future applications could include hypersonic missiles and aeroplanes, and reusable single- or two-stage-to-orbit space launchers.

Subsonic engines have rotating parts to compress air

Scramjets burn hydrogen fuel in a supersonic air stream

Air is compressed by fast forward movement of vehicle

Rapid expansion of hot air out of exhaust produces thrust

Some commentators have even speculated that scramjets could one day be used on passenger airliners, dramatically cutting long-haul journey times.

"Absolutely, we're just going to have to wait a while," said Joel Sitz, the X-43A project manager.

"It took a long time from the Wright brothers to the first flight across the ocean. It might take the same amount of time before this technology can be applied to commercial aircraft, but I think some day it will be there."

Tuesday's flight went ahead as planned over the Naval Air Warfare Center Weapons Division Sea Range, which is just to the northwest of Los Angeles.

The X-43A was positioned on the nose of a Pegasus booster rocket and attached to the underside of a B-52B aircraft's wing.

The paired X-43A and booster were then lifted to an altitude of 12km (40,000ft) and released; the booster taking the scramjet higher and faster.

Separation and onward flight of the scramjet occurred at an altitude of about 33.5km (111,000ft).

The telemetry showed the X-43A was set free by the booster at a speed well in excess of Mach 9 but was able to maintain its cruising velocity under the thrust from its scramjet.

Engineers followed the X-43A as it travelled more than 1,000km (620 miles), eventually losing speed and plunging into the Pacific.

Fast flowing

The advantage of scramjets becomes apparent once they have been accelerated beyond about Mach 4. They are able to burn their fuel - in this case hydrogen - without the need to carry heavy tanks of oxidiser, as rockets must.

Instead, they scoop their oxygen from the air, which is naturally compressed by the forward speed of the vehicle and the shape of its inlet.

A booster rocket lifted the X-43A high into the sky

Conventional jet engines have rotating blades to compress the air; scramjets have no such moving parts.

The fuel is ignited in a supersonic air stream - a technical challenge that has been likened to "striking a match in a hurricane". Successful combustion relies on critical control of temperature and pressure within the engine.

The concept should allow for greater payload capacity at reduced cost - and finer control over a vehicle.

Unlike rockets which tend to produce full thrust or nearly full thrust all the time, scramjets can be throttled back and flown more like an aeroplane. This makes them potentially safer.

Future funds

Tuesday's flight was the third and final mission for the Nasa vehicle, which came out of the agency's Hyper-X programme. It is not clear where the technology fits into Nasa's future plans.

A proposal to develop a larger hypersonic vehicle - the X-43C - was scrapped following President Bush's announcement in January that America would revive manned missions to the Moon by 2015 and attempt an expedition to Mars.

All effort is now directed at finding a suitable replacement for the space shuttle, and this is to be based on conventional rocket technology.

Nasa will be reluctant, however, to give up the gains - and the successes - of the X-43A.

"We'll work our way through what we think we have learnt and see what our next step might be, and talk about some foundational technologies we might want to do," said Dr Vic Lebacqz, the head of aeronautics at Nasa, after Tuesday's flight.

For Joel Sitz, the vision is develop vehicles that can take off from a standing start and switch to different propulsion systems as they go faster and faster.

"The next step I would like to see at Nasa is to take a turbine engine and a ram-scram-jet, combine those propulsions cycles, put some hardware together and start testing it.

"Maybe in a couple of years we could then put the budget together to put an aeroplane around that technology."

Outside Nasa, a group at the University of Queensland in Australia plans to conduct two Mach 8 flights and a Mach 10 flight in September 2005.

The Queensland team will test three separate scramjet configurations designed by the UK's QinetiQ company, the US Defense Advanced Research Projects Agency (Darpa) and the Japanese Aerospace Exploration Agency (Jaxa).

The US Air Force has a programme called HyTech, which is aimed at developing a hypersonic cruise missile.